In conventional techniques for measuring strain induced from forces applied to material, for instance torque or linear strain applications, sensors have been fastened to the material to be measured on and then an electrical coupling is established between the sensors and conditioning and analyzing electronics. This has drawbacks, for instance in a need for the electrical coupling to be maintained with a suitable quality over time, e.g. a problem in applications involving rotary parts forming the object of interest while the conditioning and analysis electronics is stationary. A typical example of such a situation is the use of strain gauges using resistive techniques measuring torque in a rotary shaft, these use slip rings to connect the sensors with electronics; the slip rings tend to degrade in quality over time due to wear of the connection interface. Strain gauges are also quite expensive and limit the applicability to certain test measurements and not specially suited for routine setups. Strain sensor may be used for detecting tension applied in materials due to mechanical forces.
For this purpose a number of different technologies have been developed in order to increase the quality and/or resolution. Sensor systems utilizing magnetic couplings in different forms, capacitive couplings, and vision based have thus been developed. Each with their own drawbacks in the form of prone to interference from external sources or error, reduced resolution, increased cost, and reduced reliability.
For instance vision based solutions often adhere to low resolution and/or require complex and expensive optical and mechanical setups. However, vision based solutions have one advantage of being more easily used in non stationary applications, i.e. where the image acquisition device does not need to be permanently fixed in a position relative the object to be measured on, but it can be removed and later (in time) brought back to measure again.
One such solution is presented in JP1131406 wherein an image acquisition device detects a pattern of Moiré fringes and determines the strain from the distance between the fringes. However, this technique requires high quality optics and can only be used for measuring strain in one direction at a time.
Another example is mentioned in WO9414029 wherein a measuring system for strain analysis of three dimensional patterned components is disclosed. This system utilizes a CMM solution with a camera measuring on a pattern formed on the object of interest. This is an expensive and mechanically complex solution in need for calibration of the pattern and which further has a low resolution.
Another solution may be found in FR2823849 using a vision based solution for measuring deformation and strain in an object.
FR 2835603 discloses a vision based solution for measurement of displacement and/or deformation of an object. However, this document does not mention the measurement of strain.
WO 2007/073272 discloses a device, method and system for determining a position of an object, and in particular a vision based solution using a pattern comprising absolute position data.
It is an object of the present invention to remedy at least one of these problems.